Laid-open specification DE 19 845 104 A1 describes a process for producing a microelectromechanical device, namely a thermoelectric transducer, produced in various embodiments at wafer level. In some embodiments, the thermoelectric components are assembled from two substrate wafers, coated with the respective complementary n/p materials, to form a component.
The microelectromechanical device (in this case a transducer) is composed of a plurality of components. It is then possible for further components (e.g. a laser diode) to be arranged on the component.
During fabrication of the thermoelectric transducer, in the final process steps the two wafers have to be patterned and soldered with respect to one another. DE 198 45 104 A1 gives a range of processes for doing this. The gold-tin system is given as a suitable solder which has to be capable of being patterned for applications in device and/or component fabrication. This solder makes it possible to achieve a soldering temperature required for the component functionality of just above the eutectic temperature of 278° C. Gold-tin is used as the standard solder for a wide range of applications in electrical engineering, partly also on account of its relatively noble character (high gold content).
One drawback of the gold-tin solder material is that this material, for use in thin-film technologies, for example in the case of sputtered contacts, has to be produced as a special target having approximately the same overall composition as the eutectic alloy. It is known that the composition of multicomponent targets changes over the course of time as a result of preferential sputtering of different elements, and consequently the basic problem of a change in the composition of the sputtered thin film is ever present.
A similar problem arises with thermal evaporation from a mixing source. A further significant drawback of this solder for the wafer/wafer bonding referred to above also results direct from the phase diagram of the gold-in system (cf. FIG. 1). Starting at the eutectic containing 70 atomic % of gold, the temperature of the solidus curve rises considerably as the gold content increases. A possible increase in the gold content in the sputtered layer compared to the target would lead to a considerable rise in the soldering temperature required. This makes process optimization more difficult.
Since the soldering operation has to be carried out at the lowest possible temperatures in order to protect the overall structure of the device, the soldering temperature must not greatly exceed the eutectic temperature of 278° C.